1. Field of the Invention
The present invention relates to a blasting apparatus used for ejecting at high speed an abrasive or grinding material composed of natural silica sand, alumina or silicon carbide powder, glass beads, fine steel balls, etc. with a fluid such as air in order to form a satin finish pattern or other pattern on a workpiece, or to perform precision engraving of glass, silicon wafers, etc., engraving of the ribs of plasma displays, coating engraving, the surface treatment such as coating pretreatment, or blasting for surface processing and, more particularly, to a blasting method and a blasting apparatus which make it possible to enlarge or modify a processing shape (herein after referred to as "processing pattern"), which is to be formed on the surface of a workpiece, by ejecting an abrasive and also to ensure uniform blast density of the abrasive in the processing pattern.
2. Description of Prior Art
Hitherto, as a suction type blast gun for this type of blasting apparatus, a blast gun 10 shown in FIG. 9, for example,has been used. The blast gun 10 is equipped with a gun main body 11 which has an abrasive intake chamber 12 into which an abrasive is introduced through an abrasive introducing inlet 24 via an abrasive hose 31 from a recovery tank of a blasting apparatus. The abrasive intake chamber 12 has a conical inner surface 16 at the front end thereof, a nozzle 14 being provided at the conical inner surface 16.
The distal end of a jet 13 having the rear end thereof in communication with a supply source of compressed air, not shown, is inserted in the conical inner surface 16 from the rear of the abrasive intake chamber 12 so that compressed air having a relatively high pressure supplied from the supply source of compressed air, may be injected through the injection outlet at the distal end of the jet 13.
A cylindrical holder 15 has a tapered inner peripheral surface. The tapered portion of the outer peripheral nozzle 14 is fitted to the tapered portion of the inner periphery of the holder 15 and the threaded portion formed on the outer periphery of the holder 15 is screwed, for example, to the gun main body 11 so as to secure the nozzle 14 to the gun main body 11.
In the blast gun 10 constructed as described above, when high pressure air is injected through the distal end of the jet 13, which is in communication with the supply source of compressed air, via the hose 32, negative pressure is produced in the abrasive intake chamber 12. The negative pressure causes the abrasive in the recovery tank, not shown, to be sucked into the abrasive intake chamber 12 via the abrasive hose 31.
The abrasive in the abrasive intake chamber 12 is drawn into an annular gap between the conical inner surface 16 and the outer periphery of the jet 13, then it rides on an air stream injected from the jet 13 so that it is sprayed while conically dispersing outside the nozzle 14 to form an approximately circular processing pattern on the surface of a workpiece.
In such a conventional suction type blast gun 10, the inside diameter of the ejection hole of the jet 13 is made small in order to permit high speed air stream released from the jet 13; therefore, the effective injection range wherein uniform processing by an abrasive ejected with an air stream, which is emitted from the jet 13 and which has a small sectional area, can be achieved is determined by the inside diameter of the ejection hole of the nozzle 14, and the processing pattern is accordingly limited.
Hence, in order to carry out blasting on a workpiece to a desired pattern, it is required to move the blast gun 10 and/or the workpiece or to take other similar measures to continue the intended pattern formed by the blast gun.
In the processing method described above, however, if a blast gun for a relatively small processing pattern is used, then the blast gun and/or the workpiece must be moved over a large area This requires relative long time to finish one processing cycle and also requires that the blast gun or the workpiece be moved accurately at a constant speed and at constant intervals in order to perform uniform processing on the workpiece, thus making the blasting difficult. For this reason, there has been a demand for developing of a blast gun which permits a larger processing pattern and also a uniform blast density of an abrasive in the processing pattern.
The suction type blast gun, however, cannot meet the demand for a larger processing pattern by using such a simply method in which the inside diameter of the ejection hole, i.e. the nozzle diameter, of the nozzle 14 of the blast gun 10 is increased.
If the inside diameter of the jet 13 is made larger to provide a larger processing pattern, then the injection speed and the injection pressure of the air stream emitted from the jet will decrease; therefore, in order to maintain the injection speed and the injection pressure at constant levels, it would be necessary to employ a larger compressor or the like with a larger capacity as the supply source of compressed air, inevitably making the apparatus larger and more expensive. If the inside diameter of the nozzle or the jet is increased to provide larger processing patterns, then the abrasive blast density in a processing pattern would be uneven, resulting in uneven blasting effects.
In view of the shortcomings of the prior art described above, the applicant has already applied the invention on a method and apparatus for increasing the width of a processing pattern by ejecting two air streams toward nearly the same positions with respect to the center of an injection stream of a mixed fluid composed of an abrasive and compressed air emitted from a blast gun in such a manner that they hold the jet stream of the mixed fluid therebetween (Japanese Patent Application No. 7-79163 applied on Apr. 4, 1995 not laid open before the convention date on which the present application is based).
According to the method and apparatus disclosed under Japanese Patent Application No. 7-79163, the processing pattern can be made significantly wider than that of a conventional blast gun and a uniform blast density of an abrasive in the processing pattern can be achieved.
The processing patterns produced by the aforesaid method is limited to circular or elliptic patterns, and it is difficult to change the processing patterns according to the material of the workpiece, the processing conditions, processing shape, etc., thus limiting the enlargement of the processing patterns.
In addition to the suction type blasting apparatus, there is a straight-hydraulic type blasting apparatus. In this straight-hydraulic type blasting apparatus, fine particles are sealed in an abrasive tank, compressed air is supplied into the tank, and the fine particles ejected through an outlet connected to the bottom of the tank are ejected with the compressed air through the nozzle; therefore, this type of blasting apparatus does not have the member corresponding to the jet of the suction type blasting apparatus, making it possible to easily enlarge the processing patterns by increasing the inside diameter of the nozzle. The straight-hydraulic blasting apparatus, however, has shortcomings such as the need for stopping the blasting apparatus to supply the abrasive when the tank has run out of the abrasive, making it unsuitable for continuous processing based on continuous blast of an abrasive. In addition, the amount of injected abrasive varies, depending on the amount of the abrasive present in the abrasive tank, leading to such problems as variations in the processing accuracy as time elapses when an abrasive is ejected continuously for a predetermined time. Thus, the straight-hydraulic blasting apparatus is unsuitable especially for forming ribs or barriers of plasma displays or precision machining and micro-machining of sapphire, glass, silicon wafer, ceramics, or other materials used for semiconductors and other electronic equipment parts.